create index with dynamically allocated blockmaps

[codecov[bot]]

Codecov Report

Merging #32 (5ddfaa5) into main (db801ad) will increase coverage by 2.15%. The diff coverage is 91.37%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #32 +/- ## ========================================== + Coverage 78.45% 80.61% +2.15% ========================================== Files 10 10 Lines 803 815 +12 Branches 176 169 -7 ========================================== + Hits 630 657 +27 + Misses 92 75 -17 - Partials 81 83 +2 ``` | [Files Changed](https://app.codecov.io/gh/lanterndata/lanterndb/pull/32?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=lanterndata) | Coverage Δ | | |---|---|---| | [src/hnsw/external\_index.c](https://app.codecov.io/gh/lanterndata/lanterndb/pull/32?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=lanterndata#diff-c3JjL2huc3cvZXh0ZXJuYWxfaW5kZXguYw==) | `82.03% <91.22%> (+4.25%)` | :arrow_up: | | [src/hnsw/insert.c](https://app.codecov.io/gh/lanterndata/lanterndb/pull/32?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=lanterndata#diff-c3JjL2huc3cvaW5zZXJ0LmM=) | `84.81% <100.00%> (+2.31%)` | :arrow_up: |

[davkhech]

LGTM!